In the pulp and paper industry, plug screw feeders are well known devices used in different applications for transporting cellulose pulp/chips in such way that a plug is formed, sealing against a higher (or lower) pressure at the output end of the feeder. The plug transport is achieved by means of a screw interacting with some kind of pipe member, often referred to as screw pipe. The screw pipe may, for example, present a tapering shape.
It is often desirable to dewater the material transported in the plug screw feeder, and openings may be provided in the screw pipe for the dewatered liquid. There can also be another pipe unit, such as a slotted plug pipe, arranged downstream of the screw pipe (in the feed direction), to prolong the dewatering zone.
In a number of known plug screw devices, straight bars are provided on the inner surface of the plug screw feeder (or the screw pipe). Similarly, straight bars may be provided on the inner surface of the subsequent pipe unit (e.g. the slotted plug pipe).
It has been proposed to make the bars of the screw pipe spiral-shaped, such as in the SpiralThroat™ product by Metso Paper. The bars are then inclined with respect to the longitudinal axis of the screw pipe, and the degree of inclination is typically the same throughout the screw pipe, or can be slightly increasing towards the end of the screw pipe. By means of screw pipes with spiral bars, an improved feeding effect is achieved. However, such spiral screw pipes result in less efficient dewatering. Furthermore, spiral screw pipes could be inappropriate for use together with known types of slotted plug pipes with straight bars.
Accordingly, there is a need for an improved plug screw feeder providing efficient feeding and dewatering of the transported pulp/chips.
One object of the present invention is to provide an improved apparatus for feeding cellulose pulp/chips. A specific object is to provide a screw pipe for a plug screw feeder by means of which both efficient feeding and efficient dewatering can be achieved. Another object of the present invention is to provide a screw pipe suitable for use together with slotted plug pipes with straight bars.